Stress/extension-measuring sensor and method for measuring stress/expansion

ABSTRACT

A stress/strain measuring sensor for the continuous monitoring of stress/strain conditions, especially in screwed bolts, along with a corresponding measuring process is disclosed. An arrangement, and a corresponding method, are provided that are uncomplicated and easy to implement, and enable a continuous monitoring of stress/strain conditions. This is attained using a sensor ( 1 ) that comprises a first inductor ( 3 ) and at least one additional element ( 2 ), which comprises at least one pressure-dependent first impedance ( 5 ) or a second impedance ( 5 ′) and a second inductor ( 3 ′), wherein the second impedance ( 5 ′) and/or the second inductor ( 3 ′) are pressure-dependent, so that when the amount of pressure applied to the element ( 2 ) changes, the resonant frequency of an electromagnetic resonating circuit ( 3, 5; 3′, 5 ′) that is formed by impedance ( 5, 5 ′) and inductor ( 3, 3 ′) changes.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a stress/strain measuring sensor forthe continuous monitoring of stress/strain conditions, especially inscrewed bolts, and a corresponding measuring process. The invention isdesigned for use, for instance, in maintenance work for the purpose ofchecking stress/strain conditions so that, for example, torque levels ofscrewed bolts can be easily monitored and adjusted.

In relation to this area of application, so-called torque keys are knownfrom the state of the art, which operate, for example, using ultrasoundsensors.

Also known are stress sensors in which piezoelectric materials are used.In such cases, the known piezoelectric effect is utilized so that, whenforce is applied to the piezoelectric material via electricdisplacement, surface charges are created. A sensor of this type isdescribed, for example, in WO 99/26046.

The problem with this, however, is that the electrical charge separationthat occurs as a result of exposure to mechanical deformation exists foronly a short time, making continuous measurement impossible.Furthermore, charge amplification is usually necessary, as is describedin WO 99/26046, in order to convert the piezoelectrically generatedcharges to a proportional stress level.

It is thus the object of the present invention to create a stress/strainmeasuring sensor and a corresponding process, which are uncomplicatedand easy to use, and which will enable a continuous monitoring ofstress/strain conditions.

The object is attained with a stress/strain measuring sensor thatincludes a first inductor and at least one other element, whichcomprises at least one pressure-dependent first impedance or a secondimpedance and a second inductor, wherein the second impedance and/or thesecond inductor are pressure-dependent, so that when the pressureapplied to the element is changed, the resonant frequency of anelectromagnetic resonating circuit formed by impedance and inductorchanges.

What is essential in this connection is that, by usingpressure-dependent electromagnetic components and by arranging them inrelation to an electromagnetic resonating circuit, the resonantfrequency of said circuit is utilized to determine strain/stressconditions. In principle, complementary components (impedance, inductor,etc.) having corresponding pressure-dependent properties can be used forthis. In the case of a pressure-dependent impedance, e.g., this would bean inductor, and vice-versa.

In contrast to the direct measurement of short-term chargeseparations—as is customarily done in the state of the art—here acontinuous measurement can be achieved via the measurement of varyingresonant frequencies. The utilization of simple, pressure-dependentelectrical components represents a particularly simple and effectivemeasuring method and enables flexible embodiments. Thus, the inventionis simple in design and easy to handle, also because no separate powersupply is necessary. In addition, only passive components are used.

According to a first embodiment, the sensor comprises a first inductoralong with an additional element that has at least onepressure-dependent first impedance. The pressure-dependent firstimpedance, with the first inductor, forms an electromagnetic resonatingcircuit, the resonant frequency of which changes when pressure isapplied to the element. Of course, the element may also compriseadditional electromagnetic components (resistors, inductors, etc.)without altering this underlying principle.

Expediently, in the first embodiment the element is comprised entirelyor partially of a dielectric material, the permeability of which changeswith the application of pressure. Advantageously this material can bewell integrated into existing assemblies because it is lightweight andsmall.

According to a preferred embodiment, the additional element of thesensor comprises at least one pressure-dependent second impedance and asecond inductor, wherein the pressure-dependent impedance and the secondinductor are connected in parallel and form an electromagneticresonating circuit, so that the resonant frequency of said circuit isshifted as the application of pressure to the element changes.

Expediently, the element in this case is comprised of piezoelectric ormagnetostrictive material. In addition, any type of materials may beused that will effect a load- or pressure-dependent electromagneticcoupling. These materials or substances can be well integrated intoexisting assemblies because they are lightweight and their dimensionsare small.

According to a particularly preferred embodiment, the sensor is designedessentially as a foil on which the first inductor is arranged, alongwith contact surfaces for contacting the additional element. A foil-typeembodiment of this kind is also advantageously characterized by alightweight design and small dimensions.

In addition, it is especially advantageous that the foil-type sensorencompasses the additional element at least partially in the area of thecontact surfaces. By bending or folding the foil-type sensor, thecontacting of the additional element can be accomplished in a multitudeof ways without difficulty.

It is further advantageous that the section of the foil-type sensor thatis equipped with the first inductor projects out above the additionalelement, which facilitates the coupling of measuring or testing devices.

It is particularly advantageous that the first inductor serves as bothcoupling an d decoupling element, so that the first inductor serves onone hand to activate the given electromagnetic resonating circuit and onthe other hand to measure the resonant frequency of the givenelectromagnetic resonating circuit. In this manner a contact-freecoupling is possible both in the activation of the electromagneticresonating circuit and in sampling the strain/stress condition. Thesensor thus requires no external leads.

In sampling the stress/strain condition it is expedient to use atransceiver as the testing device, which can be coupled to the sensorvia the first inductor.

According to a particularly preferred embodiment, the additional elementis integrated into a flat washer, which can be positioned between amounting assembly and a structure that is attached thereto. In thisembodiment as well, it is advantageous that the additional element iscontacted, for example, via a foil-type section, and that the section ofthe foil-type sensor that is equipped with the first inductor projectsout over the flat washer, so that a testing device can be easily coupledto it.

According to an alternative embodiment, it is expedient to integrate asecond element into the flat washer as a comparator element. This hasthe advantage that, in the determination of stress/strain conditions,the effects of temperature or aging can be compensated for, as onlychanges in the resonant frequency are registered.

The object stated above is further attained with a method for measuringstress/strain, which is characterized pursuant to the invention in thatat least one element of a sensor with a first inductor, which comprisesat least one pressure-dependent first impedance or a second impedanceand a second inductor, wherein the second impedance and/or the secondinductor are pressure-dependent, is arranged between a mounting assemblyand a structure that is connected to the mounting assembly such thatwhen the pressure that is applied to the element changes, the resonantfrequency of an electromagnetic resonating circuit that is formed byimpedance and inductor is changed.

What is expedient here is that the element is compressed with theapplication of pressure, and when the amount of pressure applied isdecreased, the compression is released, and that the appropriateelectromagnetic resonating circuit is activated via the first inductor.

It is further advantageous that the measurement of the resonantfrequency of the electromagnetic resonating circuit is accomplished viaa contact-free coupling to the first inductor.

According to an alternative embodiment, it is expedient, using a secondelement, to perform a comparative measurement to compensate for theeffects of temperature or aging, as only a change in the pressure/stressconditions or the resonant frequency is registered.

The invention is appropriate for use, for example, in adjusting torquein screwed bolts and thus replaces known torque keys. The invention canbe used, e.g., in maintenance work on aircraft, helicopters or othermodes of transportation.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Below, the invention will be described in greater detail with referenceto the attached diagrams. In these:

FIG. 1 shows a schematic representation of the sensor specified in theinvention for determining the stress/strain conditions of a screwedbolt;

FIG. 2 shows a plan view of a foil-type sensor;

FIG. 3 shows a perspective view of a foil-type sensor;

FIGS. 4, 4 a-c show the analogous electric circuit of the sensoraccording to various embodiments;

FIG. 5 shows a representation of the resonant frequency under differentlevels of pressure; and

FIG. 6 shows the resonant frequency as a function of the application ofpressure.

DETAILED DESCRIPTION

FIG. 1 shows a schematic representation of the sensor specified in theinvention for determining the stress/strain conditions of a screwedbolt. In FIG. 1 the sensor is indicated by the number 1 and isintegrated into a flat washer 10. The flat washer 10 with the integratedsensor 1, hereinafter also referred to as the modified flat washer, ispositioned between a bolt 11 and a structure 12 that is connected tosaid bolt. Further, a testing device 13 (e.g. a transceiver) is coupled,contact-free, to the sensor 1, which will be described in greater detailfurther below. Via a data line 14 the data obtained from the transceiverare passed on to an evaluation unit (not illustrated here).

The sensor 1 comprises a dielectric, piezoelectric or magnetostrictiveelement 2, which is indicated only schematically in FIG. 1. Inprinciple, materials with load- or pressure-dependent electromechanicalcouplings may be used. In FIG. 1 the element 2 is integrated into theflat washer 10 in such a way that its surface is arranged essentiallyperpendicular to the direction F in which pressure is applied. Theelement 2 is contacted via a foil-type section of the sensor 1, as isshown in FIGS. 2 and 3.

FIG. 2 shows a plan view of a foil-type sensor 1, in which the element 2is not visible. On the foil-type sensor a first inductor 3 is applied ina meandering form and is connected to corresponding contact surfaces 4and 7. The contact surfaces 4, 7 serve to contact the element 2. To thisend, the foil-type sensor as shown in FIG. 2 is bent around the fold orbreak point, indicated here by a dashed line, in order to contact theelement 2, as shown in FIG. 3. In this, ordinarily the section of thefoil-type sensor 1 that is equipped with the first inductor 3 projectsout over the element 2, in order to facilitate a coupling of measuringdevices (see FIG. 1). The sensor arrangement shown in FIG. 3 isintegrated into the flat washer 10, as described above. Of course, thesensor arrangement may also be integrated into other spacing orintermediate components.

FIG. 4 shows the analogous electric circuit of the sensor 1 in variousembodiments. In this, the electrical element and the first inductor areindicated by the same reference numbers as in the previous diagrams. Inaddition, in FIG. 4 the line resistor is indicated by the number 6. Ofcourse, other electrical components may also be included in theanalogous electric circuit, without affecting the underlying principleof the invention.

The electrical component 2 can be designed differently. According to afirst embodiment (4 a) the element 2 comprises a condenser with apressure-dependent impedance and is indicated below by the number 5.This is implemented, for example, with a dielectric element, thepermeability of which changes with the application of pressure. Thepressure-dependent impedance 5, together with the first inductor 3,forms an electromagnetic LC resonating circuit, the resonant frequencyof which changes with the application of pressure.

According to a second embodiment (4 b), the element 2 itself comprisesat least one impedance and an inductor connected to it in parallel,which are indicated in FIG. 4 b similarly by the numbers 5′ and 3′. Inpractical terms this is implemented using piezoelectric and/ormagnetostrictive elements 2. In this embodiment, the electromagneticresonating circuit, the resonant frequency of which changes with theapplication of pressure, is formed by the impedance 5′ and the inductor3′. In addition, the impedance 5′ and/or the inductor 3′ can bepressure-dependent. Of course, with this embodiment as well, otherparallel or series-connected components may be considered, withoutaffecting the fundamental principle.

According to a particularly preferred embodiment (FIG. 4 c), the element2 is made of a piezoelectric material. As is known, a piezoelectricelement, due to its own material state, possesses a mechanical resonanceand an inherent capacitance, and can be illustrated by the analogouscircuit shown in FIG. 4 c. Consequently, here, as in the secondembodiment shown in FIG. 4 b, the electromagnetic LC resonating circuitis formed by the impedance and/or inductor, also indicated by thenumbers 5′ and 3′, so that with the pressure-dependence of the impedance5′ a shifting of the resonant frequency with the application of pressureto the piezoelectric element 2 takes place. With the application ofpressure, the piezoelectric element 2 experiences a compression, whichresults in a corresponding charge shift (“piezoelectric effect”) and,with the material-based pressure dependence of the absolutepermittivity, thus results in a shift in the resonant frequency.

In the above-described embodiments, the element 2 experiencescompression with the application of pressure, and with a decrease in theamount of pressure applied, experiences a corresponding release of saidcompression. This in turn leads, as described above, to a measurableresonant frequency shift, so that the condition “bolt stressed” or “boltunstressed” can be continuously monitored.

An application of pressure to the element 2 thus effects, e.g., a shiftin the resonant frequency to higher frequencies, as is illustrated, forexample, in FIG. 5 and FIG. 6. When the amount of pressure applied isdecreased, the resonant frequency shifts proportionally to lowerfrequencies. Of course, an arrangement may also be selected in whichthis method is reversed.

It should further be noted that the activation of the presentelectromagnetic resonating circuit is accomplished via the firstinductor 3, which thus serves as the coupling element. This can beaccomplished contact-free (e.g. capacitively). However, the firstinductor 3 serves at the same time as an antenna or decoupling elementfor measuring the resonant frequency. Here again, the measurement ispreferably conducted contact-free.

According to a further embodiment (not illustrated here), a secondelement (e.g. made of dielectric, piezoelectric or magnetostrictivematerial) is arranged in the flat washer 10 in order to allowcomparative measurements. To accomplish this, a metrological bridge isconstructed of one mechanically stressed and one mechanically unstressedelement 2, whereby the relative displacement of the resonant frequencycan be determined. An arrangement of this type or comparativemeasurement enables, for example, a compensation for the effects oftemperature, aging, or similar factors.

Finally, it should be noted that in principle, a series of differentpossible arrangements of electromagnetic components to formcorresponding electromagnetic resonating circuits is conceivable, whichenable a stress/strain measurement that can be conducted on the basis ofthe above principle. The above-described embodiments are only exemplaryembodiments, and are not intended to limit the scope of the object ofthe present invention.

1. Stress/strain measuring sensor for the continuous monitoring ofstress/strain conditions, wherein the sensor comprises: a firstinductor; and at least one other element which is made of piezoelectricor magnetostrictive material, and which comprises at least onepressure-dependent first impedance or a second impedance and a secondinductor, wherein the second impedance and/or the second inductor arepressure-dependent, so that when the amount of pressure being applied tothe at least one other element is changed, the resonant frequency of anelectromagnetic resonating circuit that is formed by impedance andinductor changes.
 2. Stress/strain measuring sensor according to claim1, wherein the at least one other element comprises at least thepressure-dependent first impedance, and wherein the first inductor andthe first impedance form the electromagnetic resonating circuit. 3.Stress/strain measuring sensor according to claim 2, wherein the atleast one other element is made entirely or partially of a dielectricmaterial.
 4. Stress/strain measuring sensor according to claim 1,wherein the at least one other element comprises at least thepressure-dependent second impedance and the second inductor, wherein thepressure-dependent second impedance and the second inductor areconnected in parallel and form the electromagnetic resonating circuit,so that when the amount of pressure being applied to the at least oneother element changes, the resonant frequency of the circuit shifts. 5.Stress/strain measuring sensor according to claim 1 wherein the sensoris essentially a foil, on which the first inductor and contact surfacesfor contacting the element are arranged.
 6. Stress/strain measuringsensor according to claim 5, wherein the foil-type sensor encompassesthe at least one other element at least partially in the area of thecontact surfaces.
 7. Stress/strain measuring sensor according to claim 5wherein the section of the foil-type sensor that is equipped with thefirst inductor projects out over the element.
 8. Stress/strain measuringsensor according to claim 1 wherein the first inductor serves as bothcoupling and decoupling element.
 9. Stress/strain measuring sensoraccording to claim 1 wherein a testing device for checking thestress/strain condition is coupled, contact-free, to the sensor via thefirst inductor.
 10. Stress/strain measuring sensor according to claim 1the at least one other element is integrated into a flat washer. 11.Stress/strain measuring device according to claim 10, wherein a secondelement is arranged in the flat washer to allow comparative measurementto compensate for the effects of temperature and aging. 12.Stress/strain measuring sensor according to claim 10 wherein the flatwasher is positioned between a mounting assembly and a structure that isconnected to said mounting assembly.
 13. Method for stress/strainmeasurement, comprising the act of: arranging, between a mountingassembly and a structure connected to the mounting assembly, at leastone element, made of piezoelectric or magnetostrictive material, of asensor with a first inductor, which comprises at least onepressure-dependent first impedance or a second impedance and a secondinductor, wherein the second impedance and/or the second inductor arepressure-dependent, such that when the amount of pressure applied to theat least one other element changes, the resonant frequency of anelectromagnetic resonating circuit that is formed by impedance andinductor is changed.
 14. Method for stress/strain measurement accordingto claim 13, wherein the element is compressed when pressure is applied,and is released from said compression as the amount of pressure appliedis decreased.
 15. Method for stress/strain measurement according toclaim 13 wherein the electromagnetic resonating circuit projects outover the first inductor.
 16. Method for stress/strain measurementaccording to claim 13, wherein the measurement of the resonant frequencyof the electromagnetic resonating circuit is accomplished via acontact-free coupling to the first inductor.
 17. Method forstress/strain measurement according to claim 13, wherein a comparativemeasurement is conducted using a second element, so that shifts in theresonant frequency can be identified.